1. Field
Example embodiments relate to complementary metal-oxide-semiconductor (CMOS) image sensors and methods of manufacturing the same. Also, example embodiments relate to image sensors comprising transfer gates formed of transparent material and methods of manufacturing the same.
2. Description of Related Art
Photodiodes (PDs) are light receiving devices that convert optical signals into electrical signals. PDs are widely used in optical pickup apparatuses such as compact disk read only memories (CD-ROMs), digital versatile discs (DVDs), or the like or optical communications, etc. If light is incident onto such a PD, pairs of electrons and holes are generated in a depletion area. The pairs of electrons and holes are transmitted through a gate or a wire line connected to an outside environment. In other words, an amount of a current flowing in a PD varies with an optical generation rate of carriers. The current functions to convert a temporally varying optical signal into an electrical signal.
An image sensor, e.g., a CMOS image sensor (CIS), generally can be divided into a photo sensing area and a peripheral area. The photo sensing area includes a plurality of unit pixels, each of which includes a plurality of gates having a PD and a transfer gate. The plurality of gates transmits a current generated by the PD to the peripheral area. The peripheral area converts the current transmitted from the photo sensing area into a digital signal through a plurality of circuits to obtain image information.
FIG. 1A is a plan view of a related art image sensor including a transfer gate, and FIG. 1B is a cross-sectional view taken along line 1B-1B of FIG. 1A. Referring to FIGS. 1A and 1B, the related art image sensor includes a PD 20 that is formed above a semiconductor substrate 10 and is isolated by an isolation layer 16. First and second p-wells 12 and 14 are sequentially disposed on the semiconductor substrate 10. Here, ions are injected into the first p-well 12 at higher density, and ions are injected into the second p-well 14 at lower density. The PD 20 generally includes a p-type PD 21 and an n-type PD 22. A transfer gate 26 is formed on a gate insulating layer 24 and transfers optical charges generated in the PD 20 to a floating diffusion area 28 through a channel area 30 of the semiconductor substrate 10.
The amount of generated optical charge is defined as sensitivity with respect to the amount of incident light. The sensitivity is affected by a numerical aperture (NA), thicknesses of layers, potential of a substrate, etc. However, the sensitivity is reduced due to a reduction in a design rule for reducing size of a unit pixel. Thus, sensitivity is required to be improved due to the reduction of size of the PD 20 resulting from the reduction of size of the unit pixel.